The XT is a monster antenna - adding 15" height over the standard MobileTI antenna. The increased height brings along an increase in gain from 3dB to 8.5dB - resulting in an increase in signal strength by nearly 4x.
WiFiRanger describes the new antenna this way:
Increase the range and performance of your WFRBoost or WiFiRanger Mobile/MobileTi with this mobile-rated 8.5dBi antenna. We have specially designed the XT Antenna to endure the road vibration of mobile use, unlike other high-gain antennas that break down over time.
The XT Antenna adds 15" of height when compared to the original low-profile antenna. It is recommended for those who already have their Mobile/MobileTi installed on a crank-up Batwing antenna. Lowering the antenna when traveling is required due to the added height. Otherwise, attaching the Mobile/MobileTi to a ladder or extendable flagpole is recommended, as long as the XT Antenna is at a reasonable height above the roofline when traveling.
The XT Antenna costs $100 as an option if you order it when purchasing a new Mobile TI, or $149 as an upgrade kit for current customers - and you then will get $20 back if your send back your old antenna.
This appears to be a very similar antenna as the one that comes integrated into the WiFiRanger Marine, which we have currently in our testing arsenal.
If you have a way to safely retract such a large antenna while underway, this may be a very substantial upgrade for pulling in remote campground WiFi. The key will be getting it well above your roofline while parked, and safely stowed away flat while not.
You can probably find an equivalent antenna cheaper if you hunt around - the benefit of going direct with WiFiRanger is the long term support and warranty, and knowing that they have tested their designs to handle on-the-road vibrations.
If any of you MIA members try this upgrade, share your experience here.
April 2015 Update: The XT Antenna has been discontinued.